1. Field of the Invention
The present invention relates to epoxy-clay nanocomposites, and particularly to a method for preparation of epoxy-clay nanocomposites by high shear mixing.
2. Description of the Related Art
Nanoclay reinforced polymer nanocomposites have attracted considerable attention lately because the dispersion of nanometer-thin layered structure of nanoclay in a polymer matrix offers new and greatly improved properties over pristine polymers. The unique layered structure of a collection of 1 nm thin, high strength, and high aspect ratio (up to 1000) clay sheets enables nanoclays to improve polymer matrix properties at very low clay loadings. A number of clay fillers and different mixing techniques have been used to synthesize polymer-clay nanocomposites. Depending on the morphology of the resultant nanocomposites, improvements in tensile strength, modulus, fracture strength, glass transition temperature, and reduction in moisture absorption of polymers have been reported for a variety of nanocomposite systems.
The improvement in the properties of polymers as a result of clay addition depends on a number of parameters, including the type of polymer and clay, curing conditions, clay concentration, and the techniques that are used to disperse the clay into the polymer matrix. Contradictory results have been reported about the effect of clay addition on the mechanical and physical properties of epoxy. For instance, improvements in the tensile strength and modulus of elasticity, flexural and compressive strength, fracture toughness and glass transition temperature (Tg) have been demonstrated by the addition of nanoclay into epoxy matrix. However, other studies reported either no effect, or reduction in tensile strength, flexural and compressive strength, fracture toughness and glass transition temperature with the clay addition.
The properties of polymer nanocomposites are highly dependent on the resultant morphology, which illustrates the degree of clay dispersion within a polymer matrix. Based on the material types and the dispersion techniques, two nanoclay structural morphologies, i.e., intercalated or exfoliated morphologies, have been reported. In the intercalated form, matrix polymer molecules are introduced between the ordered layers of clay, resulting in an increase in the interlayer spacing. However, in the exfoliated form, clay layers are separated and distributed within the matrix. The degree of clay dispersion is believed to be the key factor responsible for the change in the properties of polymer-clay nanocomposites, and exfoliated structures are reported to possess better properties than the intercalated ones. The exfoliated morphology of polymer-clay nanocomposites yields the maximum improvement in properties because maximum reinforcement is achieved with the tearing away of the individual sheets from the nanoclay tactoids and their more effective dispersion in the polymer matrix.
There are three general ways of dispersing clay in polymers, namely, (i) the direct mixing of the polymer and clay, either as discrete phases or in solution; (ii) in situ polymerization in the presence of clay; and (iii) both in situ formation of nanoparticles and in-situ polymerization. Using in-situ polymerization, different processing techniques are utilized to disperse the clay in epoxy monomers or monomers solution, including simple mechanical mixing, ultrasonication mixing, high shear mixing, and slurry-compounding. The most common techniques that are used to synthesize epoxy-clay nanocomposites are ultrasonication mixing and shear mixing. Nanocomposites processed by high shear mixing are reported to exhibit better mechanical properties as compared to those processed by ultrasonication due to a higher degree of clay dispersion achieved by high shear mixing. In high shear mixing. the shear force reduces the size of the clay particles by splitting, and increases interlayer spacing by the forced entry of the epoxy monomer between the clay galleries. Higher mixture viscosity and high mixing speed tend to generate higher shear force during mixing, and thus provide better clay dispersion.
Thus, a method for preparation of epoxy-clay nanocomposites solving the aforementioned problems is desired.